Process for the recovery of hydrocarbons



Aug. 8, 1944. P; c. KEITH PROCESSFOR TIIE RECOVERY OF HYDROCARBONS Filed om. 2e, 1940 PERC/VHL CKE/7H INVENTOR 7 .BY @Z/w/ ATTORNEY PWN Patented Aug. 8, 1944 PROCESS FOR THE RECOVERY F HYDROCARBONS Percival C. Keith, Peapack, N. J., assignor to The M. 'W. Kellogg Company, Jersey City, N. J., a corporation of Delaware Application October 26, 1940, Serial No. 362,993

4 Claims.

This invention relates to the treatment of hydrocarbon mixtures containing normally liquid hydrocarbons, normally gaseous hydrocarbons of relatively high boiling points and lower boiling normally gaseous hydrocarbons which are relatively difcult to liquefy such as methane and ethane. More particularly, the invention relates to a process wherein a mixture of such hydrocarbons, existing under a relatively high pressure, is expanded to facilitate separation of highboiling constituents thereof, and residual constituents are recompressed to the original pressure.

An object of the invention is to provide for the recovery of normally liquid hydrocarbons and relatively high boiling normally gaseous hydrocarbons from such mixtures by a process wherein the net energy consumed in the process is at a minimum and is supplied to4 the process in a simple and economical manner.

While my invention is of general application it is particularly useful in connection with oil eld exploitation. When the drilling of a well results in the discovery of a large reservoir of natural gas for which there is no immediate market, legal restrictions may, and principles of conservation do, forbid extraction of the more desirable relatively high boiling constituents from the g'as unless the residual portion, ordinarily comprising hydrocarbons having 3 or less carbon atoms per molecule, is returned to an underground reservoir. Frequently reservoirs of natural gas are found under pressures of 2000 pounds to 3000 pounds per square inch or higher. Under these circumstances it is generally preferable to expand the gases to a pressure below 2000 pounds per square inch prior to any separation steps in order to obtain more selective separation of the constituents desired to be recovered. Separation is less selective at relatively high pressures because differences in the vapor liquid equilibrium constants of the hydrocarbons tend to diminish as the critical pressure approaches. However, the advantage of improved selectivity at lower separation pressures is obtained at the expense of recompressing the residual hydrocarbons to be returned to an underground reservoir.

In accordance with my improved process energy derived from the expansion of the original mixture from its relatively high pressure is utilized for recompressing the residual gases, a substantial portion of the energy thus derived from the expansion of the original mixture having been added thereto in the form of heat,

A more detailed explanation of one embodiment of the invention will now be given with particular reference to the treatment of high pressure well gas and apparatus suitable therefor is shown in diagrammatic elevation in the annexed drawing. It is to be understood, however, that this is chosen solely for illustrative purposes, as the invention may take many forms and is not limited to the particular ow arrangement shown.

It is usually not economical to treat well gas containing less than about .2 gallon of butane and heavier hydrocarbons per thousand cubic feet` The richer gases may be accompanied by greater or less amounts of hydrocarbons in the liquid state particularly after an extended pipe line run during cold weather, and it is to be understocd-that the term well gas as used herein may include the total iluid Withdrawn from a well, whether or not some condensation has occurred.

Referring now to the drawing, well gas under pressure is passed through line I to a heat exchanger 2 wherein it undergoes a preliminary heating step. The gas then passes through line 3 to a heater 4, which may be of any suitable type. such as a coil heated by hot products of combustion, wherein the gas is heated to a temperature in conformity with the requirements of subsequent steps of the process, as will be hereinafter explained. The heated gas under pressure is passed to a work engine 5, which may be a reciprocating expander, turbine, or the like, wherein its pressure is reduced and wherein at the same time it is caused to generate mechanical energy. Upon expansion the gas undergoes a lowering in temperature but in most cases will still be capable of helping to heat the incoming well gas, and this is accomplished in the exchanger 2. The exchanger has an additional advantage in that it assists in cooling the expanded gas to the temperature at which it is desired that the subsequent separating step shall be effected. Continuing through line l the partly cooled gas is cooled further if necessary by means of the cooler 8 and thence by line 9 is introduced into a separating drum l0 wherein, in the event that the gas is below its dew point, any accompanying condensate is separated out and Withdrawn through line H.

The cooled gas free of condensate continues through line l2 and into the base of an absorber I3. A suitable absorbent medium, conveniently one derived from the heaviest portion of the well gas itself including, for example, the Cs to C14 hydrocarbons is introduced into the top of absorber I3 by means of line I4, and in passing downwardly therethrough in countercurrent contact with the rising gas absorbs heavier constituents. Enriched absorbent is withdrawn from the base of absorber I3 through line I5 and passed to a flash drum I6 maintained at a lower pressure than absorber I3. As the pressure is lowered ex-cessive amounts of undesired light components are vaporized out of the enriched absorbent and are withdrawn from flash drum I8 through line II to be disposed of as desired.

Enriched absorbent from which excessively light hydrocarbons have been partially removed is passed via line I8 from flash drum I6 to a stripper I9, wherein absorbed constituents are substantially completely vaporized and driven out of the absorbent, the necessary heat being supplied by a suitable reboiler 20. Stripped absorbent medium is withdrawn from stripper I9 through line 2| and after cooling in cooler 22 is returned for further use in the absorber.

Recovered constituents leave the stripper I9 in vapor form via line 23 and are condensed in condenser 24. The resultant condensate travels via line 25 and joins the primary condensate passing through line II, preferably for transfer to a stabilizer or other equipment for the removal of remaining wild constituents whence a stable product may be withdrawn.

Gas which has been denuded of heavier constituents in absorber I3 passes overhead through line 26 and is fed into the inlet of a compressor 21 which again may be of the reciprocating or turbo type as desired, and recompressed to a sufficiently high pressure for it to be reintroduced through line 28 into an underground reservoir, the same as or adjacent to the one from which it was originally withdrawn. The motive power for compressor 21 is supplied through a shaft 29 or any suitable power transmitting means connected to the expander engine 5, in which the initial expansion of well gas is taking place.

Typical operating conditions will now be given for the exemplary embodiment of the invention just described. As was pointed out with reference to the flow arrangement, these conditions in no sense limit the scope of my invention.

A well gas having the following composition was availiable under a pressure of 2700 pounds per square inch and at a temperature of 120 F.:

Hydrocarbons Mol per cent C4 C5 and heavier.

.ceso

Hydrocarbons Mol per cent Hydrocarbons Mol per cent C4 C5 and heavier In absorber I3 an absorbate of the following approximate composition was picked up by the lean absorbent:

Hydrocarbons Mol per cent C1 52.0 C2- 19. 6 C: 13. 9 C4 8. 0 Cr and heavier.r 7. 5

Hydrocarbons Mol per cent UIGUI 4 1 1 4 C5 and heavier This residue gas comprised 86% of the feed to the absorber or 72.6% of the total feed gas. Therefore, for every 1000 cubic feet of feed gas expanded from 2700 to 1800 pounds per square inch through work engine 5 there were 726 cubic feet to be compressed in compressor 27 from 1800 to 300D pounds, the latter pressure having been found necessary for the return of the residue gas to the ground. At a feed rate of 1000 cubic feet per minute the power required by compressor 2l was 38 horsepower, all of which was sup plied through shaft 29 by the expansion of the heated well gas.

The power balance obtained in this exemplary operation was due to the selection of 680 F. as the temperature to which the well gas was heated prior to expansion. In the event that a higher percentage of the well gas had remained to be returned to the ground, the load on the compressor would have been greater and more energy would have had to be added to the Well gas in the form of heat prior to expansion.

The additional heat could then have been provided by heating the well gas to a higher temperature in heater 4. There will be in all cases some temperature adequate to provide sucient power during expansion for the recompression step, and as the calculation of the requisite temperature from published hydrocarbon data is within the ability of one skilled in hydrocarbon processes, it need not be enlarged upon herein. Likewise, no detailed description is given of the operating conditions employed in the stripping and subsequent steps, these being relatively independent of the energy-transferring features of the invention and susceptible of considerable variation;

There is, of course, a wide range of pressures within which the absorption step may be practiced, and it will be understood that as the pressure in the absorber is lowered, increasing amounts of power are generated in the expansion of the feed gas and consumed in the recompression of the lean gas. Economic factors will determine what the optimum absorption pressure will be under any given set of circumstances.

It will be seen that by the method of my invention, exemplied in the process described, I am enabled not only to conserve the initial pressure energy of a gas and accomplish a separation of heavy hydrocarbons from light hydrocarbons at a low pressure without having to supply inordinate amounts of energy, but also am able to supply make-up energy in place of that lost due to the inefficiency of the expander and compressor in its cheapest form, i, e., heat. In order to do this I make use of the gas undergoing treatment as a power-generating medium. Even if 100% of the energy liberated by expansion c`ould be recovered and utilized in compression, the energy added as heat would usually be needed in a reservoir repressuring operation, for a pressure greater than that existing in an underground reservoir is necessary to return gas to that reservoir. It is not to be supposed however that all the energy of recompression originates as heat supplied prior to expansion, for a large part of the necessary energy would be liberated in expanding the gas in the absence of the heating step. It is the ability to secure a power balance by adding just the right amount of heat which enhances the energy-conserving feature of my invention.

Numerous alternative schemes may be employed within the scope of my invention, such as the use of interstage heaters on the expander 5 in place of or in addition to heater 4. Such variations will readily occur to those skilled in the art, and as stated earlier, the invention is not limited to any one of the modifications disclosed.

I claim:

1. In a process for the recovery of heavier hydrocarbons from a hydrocarbon mixture withdrawn from an ungrground. reservoir, the combination comprising heating said mixture to an elevated temperature prior t'o separation of heavier hydrocarbons therefrom to materially increase the energy contained therein to a point where no additional energy is required to effect recompression of light constituents as hereinafter specified, thereafter reducing the pressure on the heated mixture, recovering energy from said pressure reduction, separating heavier hydrocarbons from said mixture, and utilizing energy thus recovered as the sole source of power to recompress light constituents remaining after separation of said heavier constituents from the mixture.

2. A process in accordance with claim 1 wherein said hydrocarbon mixture is heated to a temperature in excess of 600 F. prior to the expansion thereof.

3. A process in accordance with claim 1 wherein a hydrocarbon absorption oil contacts said heated hydrocarbon mixture after expansion thereof to effect separation of heavier hydrocarbons from said mixture.

4. In a process for the recovery of heavier hydrocarbons from a hydrocarbon mixture withdrawn from an underground reservoir, the combination comprising heating said mixture to an elevated temperature prior to separation of heavier hydrocarbons therefrom to materially increase the energy contained therein, thereafter reducing the pressure on the heated mixture, recovering energy from said pressure reduction, passing the heated mixture at said reduced pressure into an absorption zone and therein contacting it with an absorption oil to separate heavier constituents contained therein and utilizing energy thus recovered to recompress light constituents remaining after separation of said heavier constituents from the mixture.

PERCIVAL C. KEITH. 

